1. Field of the Invention
The invention relates to blind fasteners, and, more particularly, to blind fasteners providing a relatively large bearing surface.
2. Description of the Prior Art
Blind fasteners are known of a type having three-pieces, a nut, a headed core bolt and an expansible sleeve. The core bolt has a threaded shank with a breakneck portion separating the shank from a breakoff portion. The nut has a head at one end to provide axial restraint on the driving side of two or more sheets and a tapered nose at its other end which extends beyond the blind side of the sheets. The sleeve is interposed between the nut's nose and the core bolt head. The fastener is set by rotating the core bolt with a driving tool engaged with the breakoff portion while holding the nut in the sheets. The core bolt head is drawn inwardly towards the blind side of the sheets to expand the sleeve over the conical nose of the nut to force the sleeve into compressive contact with the sheets. Upon reaching a predetermined torsional stress, the breakoff or driving portion of the core bolt severs from the shank at the breakneck groove. Axial restraint is provided in the set fastener by the expanded sleeve and the head of the nut. This type of fastener is described in U.S. Pat. No. 2,765,699 to J. LaTorre.
The type of fastener just described is extensively used as a substitute for standard nuts and bolts in aerospace and aircraft applications where high speed and ease in installation are required. The nut employed has a cylindrical intermediate section which is received in the hole in the sheets secured by the fastener. A small clearance is usually provided between the intermediate portion and the hole. The clearance results from the practical inability to provide an interference fit between the cylindrical intermediate portion and its mating hole.
Such sheets may be made of many suitable materials. One of these materials used on aircrafts is a graphite composite which is relatively soft and might be crushed or permanently deformed by such prior art fasteners. That is, most blind fasteners do not provide a large enough bearing surface on the blind side.
Certain sized apertures or mating holes are generally specified by aircraft companies and thus certain diameter blind fasteners are designated. The aforementioned prior art blind fasteners that would normally be used in such pre-designated mating holes do not provide a sufficient bearing surface on the blind side.
In U.S. Pat. No. 2,787,185 to Rea, an expansion fastener having a sealing liner is disclosed. Rea thus shows a pair of sleeves 36 and 38, sleeve 36 being a sealant liner and unable to support a load. The material of sleeve 36 of Rea et al may be nylon, not an outwardly expansible ductile material capable of stretching without splitting but able to support a load when expanded. The purpose of the sealant liner of Rea is to extrude into any clearance area of the fastening system which might result in a "leak path" which would be unacceptable in a fluid-tight system. For this reason, it is not necessary that the inner diameter of sleeve 38 be of any particular configuration or of an outwardly expansible ductile metallic material. Further, sleeve 38 of Rea et al is an easily deformed or molded material and sleeve 36 is a sealant. This is because the entire purpose of Rea et al is to provide only a fluid-tight fastener, not one also capable of supporting a load.
There is thus a need for a blind fastener which can be inserted in a designated size mating hole in a material, such as graphite composite, which will provide a sufficiently large bearing surface.